Cloned DNA sequence of human terminal transferase and monoclonal human TdT antibodies will be used to explore questions directly related to the biological activity of this enzyme. These questions are: 1. Is TdT generally mutagenic in any prokaryotic or eukaryotic system that contains it? 2. Is the structure of TdT related to its cellular location and regulation of its activity? 3. Is the promoter sequence for TdT specific for tissues and stage of differentiation? 4. Is TdT specifically mutagenic in systems that normally contain it? Answers to these questions will help to understand the normal role of terminal transferase in the immunopoietic system and its abnormal expression in acute leukemias.